1 Running head: AN ERGONOMIC OVERVIEW OF EXOSKELETONS

An Ergonomic Overview on Exoskeletons, Orthosis, and

Prosthesis: Potential Impacts and Future Research Directions

The views expressed in this paper are those of the author(s) and do not necessarily reflect the

official policy of the Department of Defense, Department of the Army, U.S. Army Medical

Department or the U.S. Government.

2 Running head: AN ERGONOMIC OVERVIEW OF EXOSKELETONS

Abstract

For over 100 years, researchers and inventors have attempted to create devices that work

in parallel with the body’s muscles and tendons in order to augment them. The potential

impact of recent Exoskeleton technology on decreasing Work Related Musculoskeletal

Disorder (WMSD) injuries and their associated reduction of monetary costs is

encouraging. With any new technology however, there are potential user risks involved

with bionic exoskeletons that need to be addressed, specifically physical ergonomic and

psychological human factor risks. This paper offers an overview on ergonomic risks on

the future use of exoskeletons in an industrial environment. It provides exoskeleton

background, discusses orthotic ergonomic risks that parallel exoskeleton ergonomic risk

factors, and considers exoskeleton psychological human factor risks.

At the early stage of this budding multi-billion dollar industry (Quinn, J., 2015), the time

to make necessary exoskeleton design changes, based on scientific/medical research, is

now. However, until standards are written and testing completed, the traditional method

of employing a Hierarchy of Controls method should be used to mitigate industrial

WMSD risk.

Keywords: exoskeleton, industrial, musculoskeletal, ergonomic, WMSD

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3 Running head: AN ERGONOMIC OVERVIEW OF EXOSKELETONS

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An Ergonomic Overview on Exoskeletons, Orthosis, and 3

Prosthesis: Potential Impacts and Future Research Directions 4

For centuries, people have been faced with the challenge of caring for the injured 5

and maimed, with missing limbs and/or musculoskeletal and neuromuscular injuries 6

(Georgia Tech, 2018). This has led to the solutions of prosthetics and orthosis. Yet 7

scientific research into human locomotion, biomechanics, and the development of new 8

materials have been applied towards creating improved solutions (including prosthetics, 9

orthoses, and now exoskeletons) only within the past 100 years. Recently, this 10

undertaking’s success has led to a situation described in an article by Quinn entitled 11

Global Exoskeleton Robot Market Size at $16.5 Million will Reach $2.1 Billion by End 12

of 2021 (2015): 13

“Global Exoskeleton Market Shares, Strategy, and Forecasts, Worldwide, 2015 to 14

2021 are poised to achieve significant growth as the exoskeletons are used inside 15

rehabilitation treatment centers and at home to provide stability for paraplegics and 16

people who need gait training. Ultimately, exoskeletons will be used for the 17

rehabilitation of all patients with serious physical injuries or physical dysfunction.” 18

(p. 1) 19

The pursuit of solutions to bodily injury and enhanced healing has long paralleled 20

the desire to augment or increase the healthy body’s strength and endurance. (Herr, 21

2009, p.1) The same products of the latest medical research have been applied to 22

completing work tasks, rather than as solutions for those suffering injury. This has 23

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blurred the lines between traditional medical prosthetics, medical orthosis, and newer 24

bionic exoskeletons either powered, unpowered, or a hybrid of the two. 25

26

Background 27

The inventor Nicholas Yagn of St. Petersburg, Russia, patented earliest known 28

exoskeleton in 1890 for a device he called an “Apparatus for facilitating walking” 29

(Yagn, 1890) (Figure 1). This design utilized a giant bow spring as an energy source to 30

facilitate leg movement. Later designs utilized gasbags to store energy. The earliest 31

powered exoskeleton was in 1919 (Kelley, 1919). Called the Pedomotor, this design 32

also was to facilitate walking. As an external power source, this device utilized a small 33

steam engine worn on the user’s back. Although neither device was actually completed, 34

an unpowered design similar to Yagn’s was improved and built by the MIT 35

Biomechanics Group in 2006 (Figure 2.). The improvements focused on reducing the 36

metabolic power needed by the user, succeeding by an average of 24% in performing 37

the task of hopping (the biomechanics of hopping are similar to running). (Herr, 2009, 38

p.3). 39

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40

Figures 1, 2. Exoskeletons that act in parallel with the human lower limb for load transfer to the 41 ground. Examples are Yagn's running aid [left], MIT's hopping exoskeleton [right]. Photo from 42 Herr, H. 2009. Exoskeletons and orthoses: classification, design challenges and future 43 directions. 44

45

Compare Yagn/MIT’s small, unpowered design (called a “passive” exoskeleton) 46

with a design the public thinks about when the word “exoskeleton’ is used: a powered or 47

“active” exoskeleton. The Human Universal Load Carrier (HULC) (Figure 3) was a 48

2010 design utilizing a large metal frame, multiple electric motors and batteries as an 49

external power source. Both designs accomplished to varying degrees the goal of 50

lowering the users metabolic cost, however the HULC was ultimately unsuccessful 51

because of its size and power consumption. (Marinov, 2016) The Yagn/MIT design 52

conversely worked because of its lighter weight and better human/user interface. 53

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54

Figure 3. The Lockheed HULC. Photo courtesy of Lockheed-Martin. 55

56

Bionic Exoskeletons and Metabolic Cost 57 The primary goal for bionic exoskeleton design and function should be to reduce the 58

amount of the user’s energy used (or metabolic cost) when performing a given work 59

task using an exoskeleton compared to not using one at all. Regardless of the functional 60

goal of an exoskeleton, minimizing the user’s metabolic costs while wearing the device 61

is crucial. According to Ferris, Sawicki, and Daley (2007): 62

“Body mechanics do not relate directly with metabolic energy use. Muscle tissue 63

requires metabolic energy to develop force. The total energy consumption depends 64

on both the force and work performed during the (user’s muscle) contraction.” 65

In other words, the metabolic cost that a user pays when performing a task not only 66

consists of how much muscle force/contraction a person uses during the task (a 67

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concentric contraction). Additionally, it is also how often during a task their muscles 68

perform controlled lengthening contractions (an eccentric contraction), and how many 69

times their muscles are forcefully tensed, without significantly changing length, to 70

maintain a static posture (an isometric contraction). All of these use metabolic energy. 71

Engineers mistakenly assume that replacing a muscle’s force output (for example, a 72

bicep muscle’s contraction when lifting an extra heavy object) with an electric motor 73

can not only increase the user’s strength but make the task of lifting that extra heavy 74

object practical to include in the user’s everyday task catalog. Adding the electric motor 75

just increases the user’s force output or strength, not making their total daylong work 76

easier. Using this strategy, at the end to the day the user will have still have paid almost 77

as much metabolic cost as not using an exoskeleton and be just as tired, if not more so. 78

Methodologies for measuring metabolic cost while using an exoskeleton are 79

currently under discussion. There are traditional methods of measuring metabolic cost, 80

such as direct calorimetry, indirect calorimetry using oxygen analysis, detailed 81

questionnaires, heart rate measurement, etc. Currently one of the most promising 82

methods to predict exoskeleton metabolic impact was developed by Mooney et. al, 83

(2014), using what they call the Augmentation Factor. 84

Power Consumption 85

The second goal of bionic exoskeleton design is lower external power consumption 86

by the exoskeleton itself. This is the reason the HULC failed; development had reached 87

a point where it needed more battery power, which increased the total weight of the 88

bionic exoskeleton system, which required more batteries and lager motors to 89

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compensate, which increased the weight again, etc. into an endless loop that halted 90

research into the design (Marinov, 2016). Ferris et al. (2007) note: “Reduction of the 91

power demands of robotic exoskeletons will allow smaller, lighter designs that are 92

easier to use and more versatile.” (p. 509) 93

Discussion 94

Ergonomic Risks 95

The Occupational Safety and Health Administration (OSHA) lists seven ergonomic 96

risks that can lead to Musculoskeletal Disorders (MSDs) (Occupational Safety and 97

Health Administration, 2018). Bionic exoskeletons are susceptible to susceptible to six 98

of the seven: 99

1. Working in awkward postures or being in the same posture for long 100

periods. Using positions that place stress on the body, such as prolonged or 101

repetitive reaching above shoulder height, kneeling, squatting, leaning over a 102

counter, using a knife with wrists bent, or twisting the torso while lifting. 103

Two different risks can be parsed from this: 104

Working in awkward postures 105

Humans are notoriously bad at using and maintaining “stressless”, neutral 106

posture even though the human body is able to perform tasks better with less risk of 107

injury. A bionic exoskeleton could be used a “forcing function”, constraining the 108

user into a neutral posture for better biomechanics. For example, the existing 109

Levitate Airframe supports the upper body during tasks, helping to alleviate static 110

muscle contractions as when holding a weighted tool at arm level for an extended 111

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period. A side effect of the Airframe is that because the Airframe’s upper arm 112

supports pull the user’s shoulders slightly back, the user finds it impossible to lift an 113

object by bending at the waist: they must keep their back in a neutral upright 114

position and bend at the knees. In their meta-analysis on a Personal Lift-Assist 115

Device (PLAD), de Loose, Bosch, Krause, Stadler, and O’Sullivan (2015), noted an 116

increase in leg muscle activity evident from electromyography (EMG). 117

“The increase in leg muscle activity could be explained by the fact that 118

external forces applied by the equipment needs to be counteracted to retain 119

balance, both in static holding and in dynamic lifting activities.” (p. 5) 120

They also noted, “…subjects were observed changing their lifting technique 121

towards a more squat-like lifting pattern, which might also may be an explanation 122

for higher muscle activity in the leg muscles when wearing a passive exoskeleton.” 123

(p. 5) 124

There is also a risk for exoskeletons making an individual’s biomechanics 125

worse. The kinesiologist Steindler (1955) defined the concept of kinetic chains as 126

“links of body parts, such as the foot, ankle, knee, and hip. Each link has an effect 127

on the others.” Horbal (2009) discusses a similar situation about foot orthotics that 128

can be applied to exoskeletons: 129

“Orthotics have been compared to eyeglasses-they are not designed to cure the 130

problem, but to assist/solve the functional problem, and to help the patient’s foot 131

work better. …However, they are also often misused, and not well thought out 132

in their application… A foot orthosis is a device placed inside a shoe and worn 133

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underneath the foot that is used to help the foot and lower kinetic chain (LKC) 134

function… Orthotics can be designed to synchronize the mechanics of the LKC 135

by holding the foot as near to its optimal functional position as 136

possible…Biomechanical dysfunction often leads to alterations in weight 137

distribution and overload to the forefoot… These functional anomalies lead to 138

altered functional biomechanics in gait leading to pain.” 139

An individual worker using a bionic exoskeleton may be facing a similar misuse; 140

a bionic exoskeleton may impose forces or constrain motion in such a way that 141

alters the natural movement sequence that the individual has acquired from previous 142

activity. 143

Working in the same posture for extended periods of time 144

As mentioned in the above example, the existing Levitate Airframe supports the 145

upper body during tasks, helping to alleviate static muscle contractions as when 146

holding a tool at arm level for an extended period. However, prolonged use of this 147

or any bionic exoskeleton could also increase user muscle weakness. Eisinger, 148

Kumar, and Woodrow (1996) addresses an analogous situation using lumbar 149

orthotics: “Prolonged use of lumbar orthotics may be associated with trunk muscle 150

weakness in the population studied. Prescribers should continue to limit duration of 151

use when possible and to consider strengthening exercises when prolonged use is 152

anticipated.” (p.1) 153

2. Localized pressure into the body part. Pressing the body or part of the body 154

(such as the hand) against hard or sharp edges, or using the hand as a hammer. 155

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Workspaces and tools causing harmful contact stress have long been a concern 156

in industrial settings. Surfaces that are too hard or sharp can cause WMSDs if they 157

have excess contact with the body. Orthotic foot inserts and/or compression mats at 158

workplaces have been used successfully to alleviate contact stress from standing on 159

a hard surface. “One of the main issues with using powered exoskeletons is the 160

creation of pressure points and skin damage due to imperfect fit or components 161

sliding across the body creating shear forces.” (Marinov, 2018). To address this 162

concern, designers have used exoskeletons that are anthropometric in nature. 163

According to de Loose et al. (2016): 164

“The main advantage (of anthropometric designs) is that the footprint of the 165

exoskeleton is relatively small as it adheres directly to the body, and the 166

movements should in theory be unrestricted… exoskeletons need to apply 167

pressure on the body to function. If not carefully designed these contact areas 168

may experience discomfort and possibly injury, which may lead to user 169

reluctance to use the exoskeleton.” (p. 5, 6) 170

3. Vibration. Both whole body and hand-arm, can cause a number of health effects. 171

Hand-arm vibration can damage small capillaries that supply nutrients and can 172

make hand tools more difficult to control. Hand-arm vibration may cause a 173

worker to lose feeling in the hands and arms resulting in increased force 174

exertion to control hand-powered tools (e.g. hammer drills, portable grinders, 175

chainsaws) in much the same way gloves limit feeling in the hands. The effects 176

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of vibration can damage the body and greatly increase the force which must be 177

exerted for a task. 178

Ergonomically harmful vibrations take place in the lower hertz range 179

(International Organization for Standardization (ISO) Standard 2631-1, 1997), and 180

express themselves in either Whole-Body Vibration (WBV) or Hand-Arm 181

Vibration (HAV) injuries. If the exoskeleton system has a direct connection to the 182

vibration source (ex. tool), the system could amplify harmful amplitudes. 183

However, the bionic system could be designed to dampen these vibrations. For 184

example, the Marine-Mojo is a passive partial-body exoskeleton that “provides 185

relief from muscle fatigue which decreases the probability of injury and increases 186

the alertness of the crew on small, fast patrol boats.” (Marinov, 2015). 187

Additionally, a worker performing tasks in a non-neutral posture are more 188

susceptible to vibration risks. (Jack, Oliver, 2008). If a worker using bionic 189

exoskeleton is forced into a non-neutral posture by exoskeleton, the user could be 190

more susceptible to vibration-caused injury. 191

4. Exerting excessive force. Examples include lifting heavy objects or people, 192

pushing or pulling heavy loads, manually pouring materials, or maintaining 193

control of equipment or tools. 194

Excessive force acting on different parts of the body during work tasks has 195

long been a factor in causing WMSDs. Orthotics have been used to reduce weight-196

bearing forces to a particular body area for recovering patients (Horbal, 2009) as 197

well as able-bodied workers. An example of the latter is using a foot insert to 198

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reduce compression stress for workers who need to stand at their workstation for 199

an extended time. Likewise, exoskeletons have the potential to reduce these 200

underlying force factors associated with developing WMSD injuries. (de Looze, et 201

al., 2016) 202

As stated above, one of the main goals for exoskeletons is to reduce a worker’s 203

fatigue and metabolic cost. As Butler states, properly designed exoskeletons 204

empirically accomplish this: “shown in results of Chase’s EMG study, the use of 205

an exoskeleton PED (personal ergonomic device) helps to prevent fatigue by 206

slowing muscle contractions that lead to the decline in a muscle’s ability to 207

generate force.” (Butler, 2016, p. 36). However, the potential for injury that could 208

result though from improperly design exoskeletons that do not reduce the worker’s 209

metabolic cost could prove disastrous. More accidents and injuries happen when a 210

person is fatigued; the resulting injuries could be larger than normal workplace 211

accidents due to the increased forces involved in output of a powered, active 212

exoskeleton. 213

5. Performing the same or similar tasks repetitively. Performing the same motion 214

or series of motions continually or frequently for an extended period of time. 215

“Repetitive lifting fatigues the musculature involved and may lead to an 216

increased risk of injury.” (Godwin, Stevenson, Agnew, Twiddy, Abdoli-Eramaki, 217

and Lotz, 2009.) Workers using exoskeletons have been tested in a number of 218

measures (i.e. % MVC, EMG, subjective questioning) and have found that 219

exoskeletons decrease worker fatigue. (de Looze, et al. 2016, p. 16). While 220

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exoskeleton use can assist the human body accomplishing repetitive motions 221

without injury, particularly passive designs, the amount of time a worker spends in 222

performing these harmful motions could counter-intuitively increase because the 223

user is feeling less pain performing the repetitive motion while using the 224

exoskeleton. Human nature tells the user they can now increase the amount of 225

time doing it. Training specific to repetitive motion risk should accompany 226

exoskeletons used for this purpose. 227

6. Combined exposure to several risk factors. May place workers at a higher risk 228

for MSDs than does exposure to any one risk factor. 229

This risk is prevalent in the field, and often requires a trained specialist to 230

parse out different risks. 231

Physiological Risks 232

One potential associated risk, unrelated to WMSD risk, are potential hot surfaces. 233

Van der Vorm, de Looze, Hadziselimovic, and Heiligensetzer, (2016) commented on 234

this in reporting on the Robo-Mate project for the European Union (Van der Vorm et al., 235

2016, p. 8, 13). Even if correctly designed however, form-fitting exoskeletons, much 236

like present day military body armor, have the potential to become uncomfortably warm 237

or hot to the wearer. Hot temperatures can cause decreased blood flow going to the 238

active muscles and brain leading to fatigue. In their review of the PLAD exoskeleton 239

system, Graham et al. (2009) noted “Several workers also reported that the device was 240

somewhat hot, which had the potential to cause heat strain and reduced productivity 241

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with prolonged exposure. A lighter material with vents would go a long way in 242

increasing user comfort.” (p. 110) 243

Human Factor Psychological Risks. 244

There are two foreseeable human factor associated risks involved with using an 245

exoskeleton. The first is an overconfidence effect. This is a well-established bias in 246

psychology, in which a person's subjective confidence in his or her judgements is 247

reliably greater than the objective accuracy of those judgements. (Pallier, Wilkinson, 248

Danthiir, Kleitman, Knezevic, Stanko & Roberts, 2002). After working with an 249

exoskeleton, a user’s perception of their strength and endurance will be altered. This 250

phenomenon was mentioned in Hugh Herr’s TED talk on exoskeletons: non-disabled 251

test subjects mentioned that after using the exoskeleton their existing biological legs felt 252

“ridiculously heavy and awkward” compared to when they had the exoskeleton on. 253

(Herr, 2016). Someone attempting a task immediately after using an exoskeleton, if 254

they are not conscious that they no longer have the augmented system, could be 255

vulnerable to an overexertion injury or accident. 256

The second risk is choosing to use an exoskeleton in the first place. As stated above 257

exoskeletons have the potential to help prevent injury and reduce costs, yet if the 258

usability is not high and it does not easily fit into a worker’s everyday routine, the 259

exoskeleton will not be used: . “…Minimization of the metabolic energy expenditure 260

will improve device usability.” (Farris, et al., 2007, p. 508). Workers generally want to 261

come into their job in the morning, put on the exoskeleton, and forget about it for the 262

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rest of the day. Speaking on website usability, Jakob Neilsen of the Neilsen Norman 263

Group (Neilsen 2012) states: 264

“If a website is difficult to use, people leave. If the homepage fails to clearly state 265

what a company offers and what users can do on the site, people leave. If users get 266

lost on a website, they leave. If a website's information is hard to read or doesn't 267

answer users' key questions, they leave. Note a pattern here?” 268

This is true about usability in general, not just websites; it will not become part of 269

the human/machine system it if it is difficult to use no matter if it is a tool, personal 270

protective equipment, or piece of electronics. The usability of an exoskeleton’s human-271

machine interface critical for user acceptance and everyday use. 272

Conclusion 273

The application of scientific research been applied into human locomotion, 274

biomechanics, and the development of new materials and devices has blurred lines 275

between prosthetics, used for persons missing limbs, orthoses, a device used to assist a 276

person with a limb pathology, and an exoskeleton, used augment the performance of an 277

able-bodied person. Bionic exoskeletons used for industrial purposes have the potential 278

to have a major positive impact on occupational health. The workforce in the United 279

States is aging (Bureau of Labor Statistics, 2014); bionics could be used as an aid for 280

those aging workers to keep them physically at their jobs. (Butler, 2016, p.33, 36). The 281

disabled using bionic exoskeletons as advanced prosthetics could lead to a greater 282

number occupational opportunities. Able-bodied workers, as well as the aging and 283

disabled, could use bionic exoskeletons to enhance their performance and endurance. 284

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With any new technology, there are potential user risks involved that need to be 285

addressed, specifically physical ergonomic and psychological human factor risks. 286

Lowering user’s metabolic costs while using an exoskeleton should be the number 1 287

goal of exoskeleton design: doing otherwise invites a host of potential musculoskeletal 288

problems and injuries to the user. Additionally, the human/machine interface (i.e. the 289

individual fit and feel) of wearing an exoskeleton is of primary concern to its acceptance 290

and usability. 291

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